If I recall right, the Draenei are a high-technology race. Therefore, considering that it would be nice to use already existing resources I would recommend you to re-skin a bloodelf tank after getting the permission.
I don't know whether this is easy, but this is what comes to my mind immediatly.
Floating Crystal siege unit: http://www.hiveworkshop.com/forums/...tone-187917/?prev=search=crystal&d=icon&r=200
Melee Siege Elekk http://www.hiveworkshop.com/forums/models.php?search=elekk&d=list&r=20 / http://www.hiveworkshop.com/forums/models-530/elekk-knight-121813/?prev=search=elekk&d=list&r=20
http://vignette2.wikia.nocookie.net/concordant/images/4/4b/Defense-Ballistae.jpg Well, you just need model..
I think it should look like a glaive thrower with crystal arcane bolts or projectiles. It should not be only made of wood but with partially steel and crystalline substance..